Sexually transmitted diseases (STDs) are a major global cause of acute illness, infertility, long-term disability, and death, with severe medical and psychological consequences for millions of men, women, and children (Hafner L. et al. Mucosal Immunology (2008) 1, 116-130).
Human papillomavirus (HPV) is a group of over 170 small DNA viruses from the papillomavirus family. It is the most commonly sexually transmitted infection, and while most HPV infections are subclinical, HPV is an etiological agent for cervical cancer, the second greatest cause of mortality among cancer-affected women worldwide. HPV-associated cancers account for over 5% of the total diagnosed cancer cases internationally, and the incidence is higher in developing countries, where HPV is estimated to cause about 500,000 cases annually. The high levels of HPV exposure and health consequences indicate the importance of developing safe vaccine candidates against HPV.
Chlamydia infection is caused by the Chlamydia trachomatis (C. trachomatis) bacterium. It is one of the most commonly sexually transmitted infections worldwide, affecting approximately 215 million people internationally. In the United States, there are around 2.8 million new cases of Chlamydia each year. Chlamydia trachomatis can also cause Chlamydia conjunctivitis or trachoma, a disease that can result in blindness. Worldwide, approximately 80 million people have active infections, resulting in impaired vision and blindness for nearly 2.2 million individuals. The international health consequences associated with Chlamydia exemplify the importance of developing effective and safe vaccine candidates against Chlamydia. 
Herpes simplex viruses (HSV) are double-stranded linear DNA viruses in the Herpesviridae family. Two members of the herpes simplex virus family infect humans—known as HSV-1 and HSV-2. Symptoms of HSV infection include the formation of blisters in the skin or mucous membranes of the mouth, lips, and/or genitals. HSV is a neuroinvasive virus that can cause sporadic recurring episodes of viral reactivation in infected individuals. HSV is transmitted by contact with an infected area of the skin during a period of viral activation.
Deoxyribonucleic acid (DNA) vaccination is one technique used to stimulate humoral and cellular immune responses to foreign antigens, such as HPV antigens, HSV antigens and/or Chlamydia antigens. The direct injection of genetically engineered DNA (e.g., naked plasmid DNA) into a living host results in a small number of its cells directly producing an antigen, resulting in a protective immunological response. With this technique, however, comes potential problems, including the possibility of insertional mutagenesis, which could lead to the activation of oncogenes or the inhibition of tumor suppressor genes.